Photopolymerizable compositions suitable for producing photopolymer printing plates are now well known in the art. In U.S. Pat. No. 3,801,328, the disclosure of which is incorporated by reference, a particularly satisfactory photopolymerizable composition is described. The composition is water developable, which enables printing plates prepared therefrom to be easily developed without incurring the risk and hazards of organic solvents, and yet at the same time produces an image surface sufficiently hard to permit repeated use. The compositions thus described in U.S. Pat. No. 3,801,328 have met with significant commercial success. Nevertheless, such compositions require long exposure times to produce a plate having a photopolymerizable surface capable of retaining a fine line or dot, because the polymerization reaction is inhibited by the influence of oxygen. Additionally, such compositions require some form of pre-exposure conditioning, so that commercially satisfactory relief images can be produced on printing plates employing such compositions.
In general, two techniques have been utilized for pre-exposure conditioning, CO.sub.2 conditioning and "bump exposure". CO.sub.2 conditioning requires maintaining the photosensitive element or plate in a carbon dioxide atmosphere until immediately prior to the image exposure. The bump exposure technique requires that the photosensitive plate be subjected to a brief light exposure immediately prior to the longer duration image exposure in the presence of a negative. Irrespective of which pre-exposure conditioning technique is utilized, both require special handling which the industry would like to avoid.
In U.S. Pat. No. 4,233,391, the disclosure of which is also incorporated herein by reference, an improved composition is described for eliminating pre-exposure conditioning of the photopolymerizable composition and plates. In the compositions described in this patent, a chemical activator comprising a phosphine derivative is included. While the resulting composition eliminates pre-exposure conditioning utilizing either the carbon dioxide or the bump exposure technique, it has been found that the photosensitivity of the coated plates decreases gradually during storage, due to the instability of the phosphine activators in the presence of oxygen. Moreover, the phosphine activators have also been found to be somewhat thermally unstable. As a consequence, it is difficult to control the quality of the images resulting from the use of such plates over a period of time.
Another composition which has been proposed to enable elimination of pre-exposure conditioning is described in U.S. Pat. No. 4,209,581, the disclosure of which is also incorporated herein by reference. According to that patent, a composition is formed from a water soluble resin, a photo initiator, and a condensation reaction product of an alkylol or alkylated alkylol derivative of urea or thiourea with an N-alkylol acrylamide or N-alkylol methacrylamide in the presence of an acid or ammonium salt. Such compositions provide printing plates with excellent sensitivity as well as a shortened developing time. However, the overall quality of the plate is not satisfactory because an excessively long exposure time is required for newspaper publishers to make printing plates from the compositions. Also, while the water resistance and hardness are generally satisfactory, additional improvement in both such features is also quite desirable.